The present invention relates to the art of earth boring tools, and, more in particular, to wear inserts particularly adapted for such tools and a method for making the inserts.
Earth boring tools take many forms. An example suitable for illustration here is a rock bit. Rock bits have rotary cutters that rotate on and break up earth formation material. Shirttails shield portions of these cutters from cylindrical bore hole walls and formation cuttings. Surfaces of the shirttails and cutters quite often are subjected to harsh, abrasive environments that tend to rapidly wear the surfaces.
In the formation of a bore hole the diameter of the hole must be held to within very close tolerances. Two reasons for this are to avoid pinching of drill bits and the necessity to ream out bore holes that have been bored under diameter.
The portion of the rock bit that determines a bore hole diameter is called the gage row. The gage row is on the rotary cutters. The gage row is subject to the very abrasive environment. Consequently without protection the gage row tends to wear down to an unacceptable diameter in an unacceptable period of time. Hardened wear resistance inserts in the gage row have been used to maintain gage tolerance over longer periods of time.
Another example of wear that can quickly degrade a tool in use is in the shirttail. The wear of the shirttail from highly abrasive environments results in the necessity of shirttail or bit renewal. Hardened inserts are sometimes used on shirttails to prolong their life. Diamonds in a cement binder have been used as inserts.
Diamonds are only used where compressive stress is not too high and are therefore not usually used where the weight of a drill string would have to be borne by them. U.S. Pat. No. 1,939,991 to Krusell describes a diamond cutting tool utilizing inserts formed of diamonds held in a medium such as tungsten-carbide mixed with a flux or binder of iron, cobalt, or nickel. The purpose for using tungsten-carbide in the binder for diamonds is to prevent the medium from wearing too rapidly and to lose its grip on the diamonds. In the Krusell patent, the flux and tungsten-carbide powder are subjected to pressure such as 30 tons to the square inch, to consolidate them. The resultant consolidated block is chilled to provide recepticles for the diamonds. The diamonds and tungsten-carbide are than packed into the holes with a pressure of much less than three tons per square inch. The tungsten-carbide is said to be sintered to provide a coherent, high-strength binder for the diamonds. Because of the extremely high melting point of tungsten-carbide, it is believed that the flux or binder was sintered and the tungsten-carbide cemented. Sintering is in a neutral atmosphere of hydrogen, nitrogen argon, or the like.
The techniques described in the Krusell patent can result in a weakness in the grip that the carbide has on the diamonds. This weakness is manifested by a physical separation between individual diamond particles and the carbide matrix. Other problems include possible solution of the diamond in the carbide and possible graphitization of the diamonds.
It has long been recognized that tungsten-carbide as a matrix for diamonds has the advantage that the carbide itself is wear resistant and offers prolonged matrix life. The flux or binder of choice has been cobalt because iron based or nickel based alloys "attack" tungsten-carbide by the formation of an eta phase carbide. Eta phase carbides are brittle. See U.S. Pat. No. 3,757,878 to Wilder and Bridwell. The solution proposed by Wilder and Bridwell encapsulates carbide particles in a sheath of a metal that does not attack the carbide. After encapsulation, the desired binder is used.
In a technical paper entitled "Iron-Nickel Bonded Tungsten Carbide" by David Moskowitz (EM 71-911, Society of Manufacturing Engineers, 1971), the problem of eta phase carbide formation in tungsten-carbide and iron or nickel systems is explained. Moskowitz states that the problem can be eliminated by providing an excess of carbon over the stoichiometric requirements of tungsten-carbide. Moskowitz reports success with iron-nickel alloy binder for tungsten-carbide with an excess of carbon. He reports improved hardness and strength for 75 WC/25 (Fe-Ni) compositions, especially with the percentage of nickel in the binder of less than about 30 percent. The particle size of the tungsten-carbide of the Moskowitz study was one micron. Specimens were pressed and then sintered in a vacuum. Moskowitz does not address the problem of diamond looseness in the matrix.
Another technique casts the carbide about the diamonds in a mold. This technique destroys the mold each time. It is an expensive technique.
Invar is a well-known iron-nickel alloy noted for its very low coefficient of expansion at temperature below about 300.degree. C. Though Invar is used in this invention, its notorious low coefficient of expansion plays no role in the results achieved by the invention. The alloy is of iron and nickel and contains about 63% iron, 36% nickel, with minor amounts usually of manganese, silicon and carbon, amounting to less than 1% in all. Invar has been used in the past as a binder for diamonds to make a cemented diamond. Nickel itself is a known wetter of diamond.